Sudden drop in Internet speed

[Zebra123]

I am currently experiencing grand network problems on my one computer. Suddenly the speed can drop to next-to-nothing, making it nearly impossible to even perform a Google serach. What is strange is that some applications (like Skype and uTorrent) can keep on downloading/uploading as normal, while others, such as Messenger and my browser, litterary are offline. Like I said, this only happens on my one computer, so I suspect that it's either related to my firewall/antivirus or my wireless adapter. I am using a N300 Wireless USB Adapter, and all drivers are up-to-date. I have changed between Avast and AVG antivirus without any luck. Any advice?

The problem started occuring shortly after I upgraded the speed of my net from 1/1 to 25/25.

Thanks in advance!

EDIT: I have sucessfully ran the SG TCP/IP Analyzer, giving me these results:

« SpeedGuide.net TCP Analyzer Results »
Tested on: 2012.09.24 14:14
IP address: 94.246.xx.xx
Client OS/browser: Windows 7 (Opera 9.80)

TCP options string: 020405b40103030201010402
MSS: 1460
MTU: 1500
TCP Window: 65700 (multiple of MSS)
RWIN Scaling: 2 bits (2^2=4)
Unscaled RWIN : 16425
Recommended RWINs: 64240, 128480, 256960, 513920, 1027840
BDP limit (200ms): 2628kbps (329KBytes/s)
BDP limit (500ms): 1051kbps (131KBytes/s)
MTU Discovery: ON
TTL: 118
Timestamps: OFF
SACKs: ON
IP ToS: 00000000 (0)

"RWIN is not fully optimized. The unscaled RWIN value is lower than it should be. Also, RWIN being close to and above 65535 does not justify the header overhead of enabling TCP 1323 Options. You might want to use one of the recommended RWIN values below."

EDIT 2: The problem only occurs while I'm on a wireless connection. I guess that's relevant.

[qualitynet]

May be there is some problem in your wireless connection. You should change your device.

[DaymonH]

Have you tried changing the channel your wireless router is using to broadcast your WiFi signal, or was the channel reset after upgrading?